AIMS/HYPOTHESIS: Women with a history of gestational diabetes mellitus (GDM) are advised to control their weight after pregnancy. We aimed to examine how adiposity and weight change influence the long-term risk of developing type 2 diabetes after GDM. METHODS: We included 1,695 women who had incident GDM between 1991 and 2001, as part of the Diabetes & Women's Health study, and followed them until the return of the 2009 questionnaire. Body weight and incident type 2 diabetic cases were reported biennially. We defined baseline as the questionnaire period when women reported an incident GDM pregnancy. We estimated HRs and 95% CIs using Cox proportional hazards models. RESULTS: We documented 259 incident cases of type 2 diabetes during up to 18 years of follow-up. The adjusted HRs of type 2 diabetes associated with each 1 kg/m(2) increase in BMI were 1.16 (95% CI 1.12, 1.19) for baseline BMI and 1.16 (95% CI 1.13, 1.20) for most recent BMI. Moreover, each 5 kg increment of weight gain after GDM development was associated with a 27% higher risk of type 2 diabetes (adjusted HR 1.27; 95% CI 1.04, 1.54). Jointly, women who had a BMI ≥30.0 kg/m(2) at baseline and gained ≥5 kg after GDM had an adjusted HR of 43.19 (95% CI 13.60, 137.11), compared with women who had a BMI <25.0 kg/m(2) at baseline and gained <5 kg after GDM. CONCLUSIONS/INTERPRETATION: Baseline BMI, most recent BMI and weight gain after GDM were significantly and positively associated with risk of progression from GDM to type 2 diabetes.